It's just a lot of practice, really. You need to think about how the weight is distributed in each pose you draw. You should also understand the way muscles and joints behave.
Try going out and drawing some really quick sketches of people.

In addition to everything said above, try to think of the human for as a real simple shape of cylinders and circles. Sketch out the limbs and torso as ovals in proportion to each other and once that looks right, then go over it with a more defined body structure. Check out some anatomy for artists books and practice like crazy.

i've found that drawing cartoons (disney style) really helped my realistic figure drawing, since a cartoon simplifies the body to the bare essential shapes. but thats just me.

I remember when I was 12... I was having a hard time drawing females... and I still am but.. My dad said he would go out and buy me a playboy to help me.... LOL! Why the hell didnt I take him up on dat???? *pounds head against wall*

My bum is on the swedish! SWEDISH... SwEdIsh.. swedish!!!
Come back bAbY cause i like to saaaaay.. i've been around the world back from bom bei. Im gonna put erasor in your face, smells a bit sheatty, dont disgrace!